1. Field of the Invention
This invention relates to a liquid crystal driving circuit. More particularly, it relates to a liquid crystal driving circuit in which the ability to drive a liquid crystal panel is variable.
2. Description of the Prior Art
With the technical progress of liquid crystal displays, to enlarge the screen and to improve the display quality have become important technical themes for a liquid crystal display. For these themes, the matching between a liquid crystal driving circuit and a liquid crystal panel is very important. When a liquid crystal panel is driven by a driving circuit, there arises two kind of matchings. That is, the voltage matching and the current matching. It is well-known that the voltage matching depends on the driving duty ratio. On the other hand, for the current matching, the following should be arranged precisely to obtain the best display quality: characteristics of liquid crystal materials in a panel and wiring materials; characteristics of the connectivity between a panel and a liquid crystal driving circuit; operating environments (for example, the operating temperature, and the brightness around an operating place; and the quality of electric supply for a liquid crystal driving circuit. Presently, the precise arrangement to improve the display quality is typically carried on by characteristic tuning of liquid crystal materials. To realize this approach, however, it is necessary to prepare several kind of liquid crystal panels which are to be used for the evaluation of liquid crystal panel, and then, to choose one of them according to conditions. Thus, time and money are wasted to find suitable conditions for the selection of panels, and so, the degree of freedom in the arrangement becomes small.
A liquid crystal driving circuit of the prior art will be explained next with referring to a drawing. In FIG. 1, a schematic diagram of a liquid crystal driving circuit according to the prior art is shown. As shown in the figure, the circuit is comprised of the following: the first power line 1; the second power line 2; the third power line 3; the fourth power line 4; output switching means 5; an output selecting circuit 6; and an output terminal 7 to drive a liquid crystal. In the driving circuit mentioned above, four power lines 1, 2, 3, and 4 are connected to output terminal 7 through switching means 5 which is controlled by output selecting circuit 6. Thus, when one switch in output switching means 5 conducts, the voltage corresponding to the power line is appeared on output terminal 7 selectively to drive a liquid crystal.
The prior art driving circuit mentioned above is connected to a liquid crystal panel to compose a liquid crystal display, and select a suitable voltage which should be applied to drive the liquid crystal panel. When a voltage is selected to output from the circuit shown in FIG. 1, however, the current capacity applied to a liquid panel is decided unconditionally from the impedance of four power lines 1, 2, 3, and 4, the conduction resistivity of output switching means 5, and the resistivity of wiring between each power line and output terminal 7. Thus, there is no room to arrange the current capacity mentioned above. In other words, the driving ability for a liquid crystal is fixed.
As mentioned above, in the liquid crystal driving circuit according to the prior art, the current capacity applied to a liquid crystal panel, or the driving ability for a liquid crystal panel is fixed, and therefore, it cannot be arranged. As a result, the current matching between the liquid crystal driving circuit and the liquid crystal panel becomes very difficult. In reality, in order to carry out the current matching, characteristic tunings of liquid crystal materials should be used. However, such tunings waste time and money, and the degree of freedom in the tunings is very small.